In a pneumatic tire, a bead portion (bead core) is configured with a bead wire and a bead insulation rubber. The bead insulation rubber requires a superior adhesion toward a wire because it plays a role in bundling and integrating bead wires.
As such, a rubber composition for tire bead insulation containing a natural rubber, a styrene-butadiene rubber (SBR), a specified esterified compound, a carbon black and an inorganic filler is disclosed in Patent Document 1 (Claims, Examples). It is described that the disclosed rubber composition exhibits superior adhesion toward wires when formed into a bead insulation rubber.